This invention relates to a technique for video display and more particularly to a split screen display which is obtained by compressing independently derived images and juxtaposing them on the screen of a television monitor for simultaneous presentation.
Several approaches are known for simultaneously displaying two independently derived images on the same screen. One approach superimposes the two images. U.S. Pat. No. 3,569,966 and 4,001,499 are exemplary of this approach. Another approach is to juxtapose the two images for simultaneous display on the same screen. For some purposes superimposition is not desirable because the resulting picture would be meaningless or confusing. For example, the military requires a split screen display in its combat aircraft for training purposes. More specifically, training aircraft now employ a television camera in the cockpit which looks out of the windscreen at the view available to the pilot. In addition, it has within its field of view the heads-up display available in modern aircraft which presents important information to the pilot on the surface of the windscreen superimposed on the outside scene. In addition to the cockpit camera, the aircraft is equipped with radar. It is advantageous to the pilot, after he is back on the ground, to see the video outputs from the cockpit camera and the radar juxtaposed on the same screen. This enables him, as well as his instructor, to review and evaluate his actions and thereby learn from them.
Picture compression of what would otherwise cover the entire screen of a television monitor is obviously required in order to juxtapose the independently derived images on one screen. One technique accomplishes the picture compression by chopping off one or both sides of the picture and utilizes only the middle portion. Just enough of each picture is eliminated so that it, along with another similarly chopped picture, fit on the screen. This technique is useful when the eliminated portions of the picture are relatively unimportant, such as with separate views of the pitcher and base runner in a televised baseball game. The compressed picture eliminates part of the baseball field and other fielders from each picture but retains the most interesting elements, namely the pitcher and baserunner. However, for some cases such as in the combat aircraft environment discussed by way of example above, the entire picture is significant and, therefore, picture compression must be accomplished in some other manner. One solution has been to selectively eliminate the video information for a selected number of points along the raster scan line. U.S. Pat. No. 4,134,128 generates a given number of samples of the received analog television signal for conversion to digital form and subsequent storage in a digital memory. The stored information is then recalled and displayed. To compress the picture, every other sample of the digitized picture, for example, is eliminated and consequently not written into the memory. Thus, when the stored information is clocked out of the memory at the normal rate, a 2 to 1 picture compression is achieved. U.S. Pat. No. 4,152,719 and published U.K. pat. application Ser. No. 2,016,857 disclose alternative techniques for selectively storing less information than is available. Retrieval from memory at the normal rate then acts to suitably vary the picture size. U.S. Pat. No. 4,220,965, on the other hand, stores all the available information but, for retrieval from memory, addresses only selected memory locations. This, again, results in the use of a reduced number of digitized picture samples and provides picture compression dependent on the ratio of samples stored to samples retrieved.
Each of the above-mentioned techniques results in some information being lost from the detected image in the process of compressing the picture. This is particularly aggravated, of course, in the first-mentioned "chopping" approach. However, the second approach of eliminating selected digitized samples also can reduce resolution and picture quality. Futhermore, the latter approach requires A/D circuitry and a sizeable memory which add to the complexity, cost, size and weight of the apparatus yet still fail to retain in the compressed picture a significant portion of the detected image.